Conventionally, for power device related applications (60-2000 V), silicon based power MOSFETs or IGBTs are used. In addition to high-voltage and high-current capability, these devices should also have low on-state power losses and good switching characteristics (e.g. fast switching with minimal switching losses etc.). However, at present neither of these Si devices offers an ideal combination of the aforementioned specifications. Specifically, a Si MOSFET has very good switching characteristics but for high-voltage applications, its on-resistance becomes very high. This limits usage of Si MOSFETs only for applications that require devices with a breakdown voltage (V.sub.B) of less than 600-900 V. On the other hand, even for devices with high V.sub.B (600-2000 V), Si IGBTs have very good on-state characteristics (low forward voltage drop at high current-density). However, Si IGBTs can be used only for low-frequency applications (&lt;40 KHz) because at high switching frequencies the switching losses for IGBTs become too high for practical applications. Thus, in the present day Si technology there is no single device that can offer combined benefits of Si MOSFET (fast switching, MOS gate control etc.) and Si IGBT (low forward voltage drop for high V.sub.B applications).
Recently, for power device applications silicon carbide (SiC) has gained a lot of attention due to its large electric field strength, high thermal conductivity and reasonably high mobility. It is expected that SiC based MOSFETs would be able to offer significantly improved performance advantages over their Si counterparts. For example, unlike Si technology where MOSFETs cannot be used for applications that require devices with V.sub.B greater than 900 V, SiC MOSFETs are expected to be useful for up to 2500 V applications
Over the last five years, different power MOSFETs based on SiC technology have been demonstrated. Some of these devices have exhibited highly encouraging results in terms of the low on-state losses, high switching speeds, and high operating temperature capability. Two of the most commonly fabricated MOSFET structures in SiC are double-implanted MOSFET (DIMOSFET) and UMOSFET.
DIMOSFET is essentially a variation of the double diffusion MOSFET (DMOSFETs) that is one of the most commonly used power MOSFET structure in Si technology. In DMOSFET structures diffusion processes are used to fabricate the source and the channel regions of the device. However, due to the lack of manufacturable diffusion technology for silicon carbide, DMOSFETs cannot be fabricates in SiC. Thus, in a double-implanted MOSFET structure (DIMOSFET) the source and the channel region are fabricated by using ion-implantation schemes. An alternative, vertical structure for silicon carbide is the UMOSFET disclosed in U.S. Pat. No. 5,233,215, entitled "Silicon Carbide Power MOSFET with Floating Field Ring and Floating Field Plate" and issued Aug. 3, 1993. The advantage of the UMOSFET structure is that it can be fabricated using as-grown epilayers and thus the the inversion channel is formed by an MOS gate along an trench etched into an epilayer.
The major problem with SiC MOSFETs based on DIMOS or UMOS technology is that due to the large breakdown field strength of SiC, the electric field in the gate oxide is very high in these devices. Experimental studies suggest that due to the high-temperature oxide reliability concerns in SiC MOS devices, the electric field in the oxide should be contained below 4 MV/cm. However, this would require limiting the electric field in SiC drift region to be much below the inherent breakdown field strength of the material. This suggests that for the case of SiC MOSFETs based on DMOS or UMOS technology, the device performance (breakdown voltage, on-resistance etc.) will be determined by the gate oxide reliability concerns and not due to the intrinsic properties of SiC.
Accordingly, it would be highly advantageous to have a manufacturable MOSFET with low ON-resistance, good switching characteristics (e.g. switching times, etc.), low leakage currents, high channel density, etc.
It is a purpose of the present invention to provide a new and improved MOSFET.
It is another purpose of the present invention to provide a new and improved MOSFET which can be fabricated in a silicon carbide material system.
It is a further purpose of the present invention to provide a new and improved silicon carbide MOSFET with lower ON-resistance, better switching characteristics, lower leakage current, and higher channel density than similar silicon devices.
It is yet another purpose of the present invention to provide a new MOSFET structure that minimizes the electric field in the gate oxide and thus, alleviates the concerns of the gate-oxide reliability at high-temperature and high electric field.